Question

The pH of $0.001{\text{ M}}$ solution of ${\text{HCl}}$ is:
A) 1
B) 3
C) 4.0
D) 5.0

Answer 1

We know that the degree of alkalinity or acidity of a solution is known as its pH. pH is the negative logarithm of the hydrogen ion concentration. We are given the concentration of ${\text{HCl}}$ solution. from the concentration of ${\text{HCl}}$ calculate the concentration of hydrogen ions. Then calculate the pH.

Complete step-by-step answer:
If the pH of the solution is less than 7 then the solution has acidic nature. If the pH of the solution is more than 7 then the solution has basic nature. If the pH of the solution is equal to 7 then the solution is neither acidic nor basic it is neutral in nature. The pH of water is 6.6 and thus, the water is slightly acidic.

We are given that the concentration of ${\text{HCl}}$ is $0.001{\text{ M}}$. We know that ${\text{HCl}}$ is a strong acid and dissociates completely in water. The dissociation reaction of ${\text{HCl}}$ is as follows:
${\text{HCl}} \rightleftharpoons {{\text{H}}^ + } + {\text{C}}{{\text{l}}^ - }$
The concentration of ${{\text{H}}^ + }$ is equal to the concentration of ${\text{HCl}}$.
Thus, the concentration of ${{\text{H}}^ + }$ due to ${\text{HCl}}$ is $0.001{\text{ M}}$.
We know that pH is the negative logarithm of the hydrogen ion concentration. Thus,
${\text{pH}} = - \log [{{\text{H}}^ + }]$
Substitute $0.001{\text{ M}}$ for the hydrogen ion concentration. Thus,
${\text{pH}} = - \log \left( {0.001{\text{ M}}} \right)$
${\text{pH}} = - \left( { - 3} \right)$
${\text{pH}} = 3$
Thus, $0.001{\text{ M}}$ solution of ${\text{HCl}}$ is 3.

Thus, the correct option is (B) 3.

Note: The concentration of hydrogen ion is known as the pH. The pH is calculated as the negative logarithm of the hydrogen ion concentration. As the calculated pH is less than 7, the $0.001{\text{ M}}$ solution of ${\text{HCl}}$ is acidic in nature.